This will: rewind the tape, show if it's really at BOT (beginning of tape), go forward by one file (one backup hopefully, it should take some time depending on how big it is), and show again how far did you get, and then rewind again. This will confirm the tape has some actual data.Very likely the restore tool doesn't work because your /dev/tape or /dev/nrtape don't work properly.

"not enough space" means there's no room to store the block read, given the blocksize you specified (i.e. the blocksize on tape is larger than 512b). There is no corresponding error to specifying a larger blocksize than exists on tape.